This invention relates to the packaging art, and more particularly, to an improved hermetically sealed press-formed paperboard tray.
Numerous types of trays made from plastic coated paperboard have been employed in the past for a variety of purposes, including the housing of moist products, sterilized materials, and food products. Such trays have been formed from unitary, plastic coated paperboard blanks using a either a drawing process or a process wherein the tray is formed by folding side and end walls of the blank along score lines and adhesively securing flap portions in the corners of the tray. The tray includes an outwardly extending flange around the entire upper edge which provides the dual function of rigidifying the tray and providing a flat surface to which a plastic cover sheet or film may be bonded in order to enclose the top of the tray after the tray has been filled.
In the case of trays formed by a drawing process, where a single sheet of paperboard is used, it is often necessary to prescore the blank of paperboard stock from which the trays are drawn at the corners thereof in order to produce evenly distributed folds in the paperboard at the corners during the drawing process. These folds or creases reduce stress concentrations in the corners while the paperboard is being formed during the drawing process and thereby eliminates potential tearing of the paperboard stock during the drawing process. The folds in the paperboard created by the prescoring thereof are present in the sidewalls of the tray and extend into the flange. The folds created in the flange result in alternating ridges and depressions forming a ribbed effect in the flange at the corners of the tray. The ribbed surface of the flange at the corners of the tray creates difficulty in bonding and complete sealing of the cover film around the entire periphery of the flange. Incomplete sealing of the cover sheet to the flange may result in some loss of a liquid product from the tray during shipping and handling and may cause degradation of a sterilized product due to exposure thereof to the surrounding environment.
Similarly, trays formed by folding a blank and adhesively securing flap portions at the corners of the tray exhibit indentations or wrinkles on the surface of the flange at the corners of the tray. Such indentations or wrinkles result when the flap portions forming the tray and/or flange corners do not conform exactly to the score lines provided in the blank. A surface indentation on the flange will also occur whenever adjacent corner flaps forming the flange are adhesively joined in overlapping relation.
Accordingly, the folded or deep drawn tray of the present invention is coated with a hot melt or wax on the edges thereof or along the depressions, indentations and/or wrinkles formed in the flange to seal the depressions, indentations and/or wrinkles to provide a leak-proof container. The hot melt or wax coating is applied to the tray flanges by a roller coater or patterned to the cover film shortly before sealing. When the film cover is applied to the flange of the tray, the tray will be hermetically sealed at the corners. A suitable release agent can also be precoated on the flange portions in the corner of the tray so that upon peeling of the cover film from the tray flange, the hot melt or wax will peel off the tray with the cover precluding possible contamination of the material housed within the tray.